1. Field of the Invention
The present invention relates to a silicon wafer etching method capable of eliminating mechanical damage due to machining, such as slicing, chamfering, and lapping, in a silicon wafer manufacturing procedure, as well as to a silicon wafer etchant for use in the method.
2. Description of the Related Art
Conventionally, silicon wafers, from which are manufactured integrated circuits such as ICs and LSIs and discrete semiconductor devices such as transistors and diodes, are manufactured in the steps of: slicing a single crystal obtained by the Czochralski method (CZ method) or the floating zone method (FZ method) through use of an inner diameter slicer or a wire saw; chamfering the peripheral edges of the sliced wafers; lapping the main surfaces of the wafers through use of free abrasive in order to improve flatness; wet-etching the wafers in order to eliminate mechanical damage induced from machining in the preceding steps; and mirror-polishing the etched wafers.
Wet etching to eliminate such mechanical damage from wafers is classified into acid etching, which uses a mixed acid consisting of, for example, hydrofluoric acid, nitric acid, and acetic acid; and alkali etching, which uses an aqueous solution of alkali such as sodium hydroxide, potassium hydroxide, or the like.
Acid etching can control an etch rate and the state of an etched surface through variation of the composition of a mixed acid, but the etch rate is generally high, thus involving difficulty in maintaining wafer flatness which has been improved through lapping.
On the other hand, alkali etching is advantageous for obtaining good flatness of etched wafers since the etch rate is relatively low. In recent years, a very high degree of flatness has been required of silicon wafers. Accordingly, alkali etching has been widely used because of the advantageous feature.
However, an alkali etchant acts to determine the reaction rate and has etch selectivity in which the etch rate of a (100) plane is 60-100 times that of a (111) plane. Accordingly, alkali etching involves a problem of the formation of roughness on an etched surface caused by this etch selectivity.
Specifically, when the roughness of the etched surface is large, projected portions thereof cause a problem that they chip off during manufacture of devices to become particles, while depressed portions require a high degree (or large amount) of polishing, resulting in reduced productivity in a polishing step.
Particularly, since the circumference of a wafer is usually chamfered through use of fixed diamond abrasive grains, in the subsequent lapping step, the chamfered portion is damaged due to contact between a carrier used to hold the wafer and the edge portion of the wafer. As a result, if this wafer is subjected to alkali etching, the surface roughness of the appropriately chamfered portion deteriorates.
In recent manufacture of LSIs, in order to prevent particle generation from the chamfered portions of wafers, there have been used wafers whose chamfered portions are mirror-polished through use of polishing pad. An alkali-etched wafer requires much more time to mirror-polish a chamfered portion thereof as compared with an acid-etched wafer having a feature of relatively low surface roughness, so that alkali-etched wafers have a problem of low productivity in mirror-polishing chamfered portions thereof.
The chamfered portion of an alkali-etched wafer has not only a problem of high surface roughness but also a problem of large dispersion (variations) in surface roughness. Since a stock removal for mirror-polishing a chamfered portion must be set to a maximum value of surface roughness, large dispersion in surface roughness causes an increase in polishing time and consumption of polishing pad or polishing agent, resulting in reduced productivity and increased cost. Thus, there has been demand for reduction in not only surface roughness but also dispersion thereof.